Numerous polyethylene compositions for the production of pipe are known. Pipe materials are classified such as PE80 or PE100. The service temperature for PE100 is 20° C. The ISO9080 classification guarantees that a PE100 material will have a lifetime of at least 50 years at 20° C. using internal stress of 10 MPa.
For many applications, it is necessary to connect different pipe segments by fittings specially adapted to the diameter of each pipe segment. In contrast to pipes, these fittings may have a complex three-dimensional structure, e.g. a T-shaped, cross-shaped or bent structure. However, fittings can also be prepared in the form of pipes of constant or varying diameter. For preparing polymeric articles having a complex three-dimensional structure, injection moulding is preferred to extrusion since it is a cost-effective means for producing these articles at high production rate. However, injection moulding requires rheological properties specifically adapted to this method, whereas the final articles should still have excellent mechanical properties, i.e. mechanical properties still meeting the PE80 or PE100 quality standards established for pipes. This is particularly true for pipe fittings, since the same long-term internal pressure does not only put a strain on the pipe, but also on the fittings connecting these pipes. Thus, quite often a polymer suitable for extrusion cannot be used for injection moulding, either due to its inappropriate rheological properties or due to insufficient mechanical properties of the resultant product.
Injection moulding is a repetitive process in which a polymeric material is melted and injected into a mould cavity where the article is cooled down. In the injection moulding process, filling of the mould and solidification partially take place at the same time. However, instant cooling generates internal stress, thereby increasing the risk of stress cracking. Thus, the polymeric melt must have sufficient flowability to fill the complete cavity before solidification takes place. Furthermore, if rheological properties are not carefully fine-tuned, the resultant moulded articles will have surface defects, e.g. stripes or wave patterns. Desired surface properties are shininess as well as a surface free from flow marks.
The step of filling the mould cavity is normally followed by a packing step, wherein a packing pressure is applied to ensure complete filling. After solidification, the mould opens and the article is ejected. However, another problem typically arising from injection moulding processes is shrinkage of the resultant article. Especially for fittings which are specially adapted to the size of other articles like pipes, very low shrinkage is of crucial importance.
EP 1 655 335 discloses a polyethylene composition with improved rheological and shrinkage behavior which is suitable for injection moulding applications, and especially suitable for fittings. However, the mechanical properties such as pressure resistance still need to be improved.
There is still a need for polyethylene compositions suitable for injection moulding applications, which show improved mechanical properties such as slow crack growth resistance and rapid crack propagation resistance, which are especially necessary for the production of pipe fittings.